The present invention relates to an improvement of time-axis compression-expansion devices for sound signals.
Tape recorders have been used for the preparation of conference minutes, of the grasp of conference proceedings through repetitive listening to the tape, for language training, etc. However, with them a number of disadvantages have been experienced, as follows, from the standpoint of efficient execution of jobs. In preparing conference minutes or in language training, the tape recorder needs to be stopped from time to time for the purpose of time adjustment, since the speed of speech coming from the tape recorder exceeds the writing speed of the listener. In addition, when one wants to run the recorder to confirm what has been recorded at a conference or lecture meeting, one needs to spend as much time listening to the tape required for recording. These jobs may be performed with improved efficiency, for example, by adjusting the motor driving system of tape recorder to slow down or speed up the reproduction speed. While such adjustment may change the speed of speech to the level appropriate for the purpose intended, it causes the pitches of speech sounds to be varied with a result of lowered intelligibility of sounds making it extremely difficult to grasp the content of speech. There has, therefore, been a demand for a device which may adjust the reproduction speed or recorded matter without an accompanying change in sound pitch. Such demand has previously been satisfied the use of rotatary magnetic head systems which handle sounds as analog signals, i.e., with no A/D conversion, with device which employs analog or digital memory elements to adjust sound pitches by altering the speed of address change for write and read, or with a device which employs a random access memory with write and read controls as exemplified by U.S. Pat. No. 3,786,195 to Schiffman. However, these conventional devices are handicapped by complex structures which make these products expensive.
Conventional devices designed to meet such demand are provided with two operating sections, one of which changes the compression-expansion ratio and the other turns on and off the time-axis compression-expansion function. For the changeover of the compression-expansion ratio two systems have been proposed, the continuous setting system and the stepwise setting system for the ratio. Treatment of sound signals with a time-axis compression-expansion device, however, will inevitably result in output of sounds with reduced quality, for example, since transfer among memory elements causes wave form to be distorted, AD or DA conversion causes noises to be made, and the discontinuity at signal connecting sections gives rise to noises. On the other hand, when sound signals are subject only to change of reproduction speed without time-axis compression-expansion processing applied, it has experimentally been proved that, although the frequency pitch is caused to change with resulting change in tone, the intelligibility of reproduced sounds remains unchanged as long as there is little difference between the reproduction and original recording speeds.
In addition, conventional devices are operated, for changing the speed of reproduced sounds, by handling by hand a knob provided on their adjustment operating sections, which manual operation is tedious and thereby lowers job efficiency in making typed or written documents from recorded tapes, etc.